The present invention relates generally to the field of disc drive data storage devices. More specifically, the invention relates to providing an electrical path between a hub and a shaft of a disc drive motor used for spinning the discs in a disc drive.
Disc drive data storage devices of the "Winchester" type are well known in the industry. Such devices utilize rigid discs coated with a magnetizable medium for storage of digital information in a plurality of circular concentric tracks. This information is written to and read from the discs using a transducing head mounted on an actuator mechanism which moves the head from track to track across the surface of the disc under control of electronic circuitry. The discs are mounted for rotation on a spindle motor which causes the discs to spin and the surface of the discs to pass under the heads.
As industry demands for smaller sizes of disc drives has lead to the introduction of half-height five and one-quarter inch drives, three and one-half inch, two and one-half inch, 1.8 inch and smaller drives, the size and configuration of the spindle drive motor has also changed. "Pancake" spindle drive motors have been used which employ an air gap in the motor which was positioned axially to the motor shaft. "In-hub" spindle motors used motor coils and magnets which were located inside the disc-carrying hub. Also, under-hub motors are common for disc drive spindle motors. These configurations may be either of a rotating or a stationary shaft configuration.
A conductive path should be provided between the storage disc and hub assembly and the drive shaft of the motor. This path prevents charge build-up on the storage disc which can particularly be a problem when using thin film and, more recently, magnetoresistive readback heads. U.S. Pat. No. 5,140,479, issued Aug. 18, 1992, to Elsing et al. entitled STATIC ELIMINATOR FOR DISC DRIVE SPINDLE describes a device used to reduce static build-up on the storage disc. In the Elsing reference, a spring-loaded conductive button provided a conductive path between the spindle hub and electrical ground. The interest is in providing a ground controlled path from head to disc. The disc is connected to the motor rotor through conductive spaces. The head is electrically connected to the actuator arm and the motor base.
In general, ferrofluid seals have been used for providing a seal to a disc drive motor to provide a barrier which prevents particles from entering the disc environment. Such a seal is shown in, for example, U.S. Pat. No. 5,011,165, issued Apr. 30, 1994, to Cap, entitled SEALING DEVICE ESPECIALLY FOR HARD DISK DRIVES.
Typical hard disc drives have used thin film magnetic heads for reading and writing information. In such a system, the electrical resistance between the magnetic storage disc to the magnetic head through the hub, motor base and arm is important. However, with the advent of magnetoresistive heads, the electrical resistance of this path needs to be more precisely known and relatively stable. In the prior art, the bearings and ferrofluid seals have provided an electrical path between the rotor and electrical ground. However, the electrical resistance of the bearings has been unstable and varies over time, temperature variations and voltage. The increase in resistance as the grease channels with time is on the order of two or three times and more. The voltage breaks down across the bearings at about 2.5 to 5 volts due to breakdown of the insulator capability of the grease. Such breakdown could damage the head. The ferrofluid seal appears as a conductor in parallel with the conductance of the bearings. The ferrofluid seal typically has a comparable electrical conductance relative to "new" bearings. Typical resistance values are on the order of 25 M.OMEGA..
The art lacks an adequate mechanism for providing a controlled electrical path between the rotor and electrical ground in a disc drive motor.